Plasmas are used for a variety of purposes in the fabrication of semiconductor devices, such as integrated circuits, and other types of substrates, such as micro-electro-mechanical (“MEMs”) substrates to achieve a variety of results. Plasma methods include the formation of a layer using plasma-enhanced chemical vapor deposition and etching techniques, such as reactive ion etching. A plasma might also be used to clean a processing chamber, or to prepare a surface of a substrate for a subsequent process step, such as a plasma wafer surface clean or activation prior to formation of a layer on the surface. Plasma generators are also used as a source of ions for ion implantation or ion milling. A directed plasma might also be used as a plasma torch for cutting applications.
The wide application of plasma processing has resulted in a wide variety of plasma processing systems and apparatus. One type of plasma processing chamber places the wafer on an electrode of the plasma circuit, opposite another planar electrode, and capacitively couples high-frequency electrical power to the two electrodes to form a plasma between them. Such a plasma reactor has advantages where it is desirable to form the plasma in the presence of the substrate, such as when the physical movement of plasma species to and from the substrate is desired. However, some devices or materials might not be compatible with this type of plasma formation, particularly the bombardment by plasma species, including high-energy photons, and associated heating of the substrate.
Another approach to plasma processing generates plasma in a remote location, and couples the plasma to a processing chamber. Various types of plasma generators have been developed, including magnetron sources coupled to a cavity, inductively coupled toroidal sources, microwave irradiation directed at a plasma precursor, electron-cyclotron resonance generators, and others. Remote plasma techniques offer a number of advantages for certain types of processes, such as cleaning deposition chambers, but generally the plasma that eventually reaches the chamber is of relatively low density, due to recombination of the reactive plasma species with each other or with components of the processing system, such as the chamber walls or delivery conduit.
Inductively coupled plasma systems have been developed that can generate a high-density plasma in one portion of the processing chamber (e.g. above the wafer), yet shield the wafer from the more deleterious effects of the plasma generation process by using the plasma itself as a buffer between the wafer and the plasma generation region and typically relies on diffusion of plasma to provide a uniform ion density across the wafer surface. In one system, a dielectric dome, or chamber top, has a conductive coil wound around the dome. High-frequency electric energy provided to the coil couples to a plasma precursor gas in the chamber and converts the precursor to plasma. In some systems, a second power supply couples an alternating field to the wafer or wafer support structure, and allows a directional component to and from the wafer to be added to the plasma generated by the coils. Such systems are used for both deposition and etch processes to achieve very desirable results, generally providing both high rates and good uniformity across a wafer.
However, the fields generated by the coil through the dome have an electric field component normal to the surface of the dome that causes plasma species to be directed to and from the inner surface of the dome. This field component acting on the plasma can cause physical erosion (“sputtering”) of the inside of the dome, as well as affect the power coupling to the plasma, thus causing a non-uniform plasma density. In some instances the plasma might contain species that react with the material of the dome, further eroding the dome and potentially creating particles than can fall from the dome onto the wafer, creating defects. Reaction of the dome material with the plasma often arises in an etch process when the material being etched is similar to the material of the dome, e.g. silica-based glass. If erosion of the inner surface of the dome continues to a point where particulate contamination or strength of the dome is an issue, the dome might have to be replaced, affecting throughput of the plasma system, and potentially disrupting the product flow through an entire fabrication line.
Transformer plasma sources have also been developed using a toroidal core. The core is typically a ferrite or similar high-permeability material, and the plasma source acts generally like an alternating-current (“AC”) transformer. Primary windings are wound around the core and an induced plasma flux around the core acts like a secondary winding(s), the plasma flux providing a secondary current to oppose the magnetic fields in the core. In one system, a tube structure forms a continuous closed path (“loop”) that includes a leg through a center opening of the core for transformer-coupled plasma. Another leg includes a gas inlet, and the same or another leg provides a plasma/gas outlet. In another embodiment, one leg of the plasma loop includes the gas inlet, gas/plasma outlet, and a process wafer. Plasma formed in the loop is carried past the wafer surface by the gas flow from the inlet to the outlet.
However, recombination of plasma species on the surface of the tubes or in the gas/plasma mixture can reduce the effectiveness of a plasma source. Recombination generally occurs to a greater degree where the distance between the plasma core, where the fields that generate the plasma are generally higher, to the process chamber are greater. Recombination can also affect plasma density, as can dilution with a process gas stream. When performing a plasma or plasma-assisted process on a wafer surface it is generally desirable to have a uniform plasma so that the surface of the wafer is uniformly processed. Uniformity problems are generally greater with larger-sized wafers.
Thus, it is desirable to provide a plasma system that avoids the surface erosion problem of conventional systems while creating a high-density, uniform plasma.